The present invention relates to manual air displacement pipettes and, more particularly, to an ergonomic, precision, low operating force, manual air displacement pipette which is free of any return spring and the operating forces associated therewith.
U.S. Pat. 3,827,305 and 4,909,991, for example, describe commercially available single channel manual air displacement pipettes. Each such pipette includes an elongated hand-holdable pipette body housing an upwardly spring biased plunger unit. The plunger unit is supported for axial movement in the pipette body between upper and lower stop positions.
In use, a pipette user grips the pipette body with his or her thumb over an exposed upper end of the plunger unit. Downward thumb action on the plunger unit moves the plunger unit downward from its upper stop position against the upward bias of a return spring to the lower stop position at which all fluid is expelled from a tip secured to the pipette. Adjacent the lower stop position is a xe2x80x9chomexe2x80x9d position for the plunger unit to which the plunger unit is returned by the pipette user at the beginning of each aspiration operation with the pipette.
In the commercially available pipettes described in the foregoing patents, the home position is defined by a xe2x80x9csoftxe2x80x9d stop. As described in such patents, the soft stop comprises a relatively stiff xe2x80x9cblow outxe2x80x9d spring mechanism within the pipette body which is activated when the plunger unit reaches the home position. In this regard, and as depicted in FIG. 9a herein, as the pipette user manually moves the plunger unit from its upper stop position by pressing downwardly with his or her thumb on the exposed end of the plunger unit, the pipette user can xe2x80x9cfeelxe2x80x9d an increased resistance to movement of the plunger unit associated with an activation of the blow out spring assembly opposing further downward movement of the plunger unit. The position of the plunger unit where the user feels the activation of the blow out spring mechanism defines the home position for the plunger unit. Continued movement of the plunger unit beyond the home position to the lower stop position is resisted by a combination of the return spring and the blow out spring mechanism.
The above described accurate sensing of the start of the increase in the downward force required to move the plunger unit is a delicate operation requiring great care to be exercised by the pipette user. Thus, with his or her thumb on top of the exposed end of the plunger unit, the user very carefully senses and then manually maintains the plunger unit at the home position. In practice, a significant portion of the total time associated with a pipeting operation is occupied by the pipette user manually maintaining the plunger unit at the home position ready for insertion of a tip extending from the pipette into a liquid which is to be aspirated by the pipette. Then, with the tip inserted in the liquid, the user manually controls the rate of return of the plunger unit from the home position to the upper stop position to aspirate a selected volume of the liquid into the tip secured to the pipette.
For accuracy and repeatability of operation of the pipette, it is important that the pipette user always bring the plunger unit to the exact same home position and that the pipette user manually control the rate of return of the plunger unit to the upper stop position in a repeatable manner for each pipette operation. This is necessary in order that the same desired volume of liquid will be drawn into the pipette tip during each repeated operation. It should be appreciated that such manual operation of a pipette places substantial physical and mental strain upon the pipette user over the course of a series of pipette operations wherein repeatability of operation is essential. In extreme cases, the physical hand and wrist strain associated with extensive and prolonged manual pipette operation can contribute to or produce repetitive strain injuries such as tendonitis and carpel tunnel syndrome.
To reduce the operating and static forces associated with commercial manual pipettes and to reduce the risk of repetitive strain injuries, the assignee of the present invention has recently developed and commercially introduced new manual air displacement pipettes incorporating latch mechanisms operable at the home position and magnet assist mechanisms operable as the plunger unit of a manual air displacement pipette approaches the home position to aide in locating and maintaining the plunger unit at the home position. Such new pipettes are fully described and illustrated in U.S. Pat. Nos. 5,364,596 and 5,700,959 assigned to the assignee of the present invention and incorporated herein by this reference.
To further reduce the operating and static forces associated with manual air displacement pipettes, employees of the assignee of the present invention have just developed a blow out springless air displacement manual air displacement pipette including a mechanical assist for aiding in the locating and maintenance of the associated plunger unit at its home position. That development is described and illustrated in the concurrently filed patent application, Ser. No. 09/522,256, filed Mar 9, 2000. assigned to the assignee of the present invention.
To still further reduce the operating and static forces associated with the operation of a manual air displacement pipette, and pursuant to the present invention, a new highly ergonomic pipette has been developed which eliminates the return spring included in all prior manual air displacement pipettes and which in certain embodiments also includes a very weak blow out spring or, in the alternative, eliminates entirely the blow out spring included in all commercial manual air displacement pipettes.
As will be described hereafter, the elimination of the return spring places the plunger unit of the manual air displacement pipette of the present invention under the total control of the pipette user who can then with minute precision and with the use of minimal thumb or finger forces accurately control the upward and downward movement and location of the plunger unit during both liquid aspiration and dispensing operations, all free of the continuous upward forces of a conventional return spring. So precise is the operation of the pipette that even the tip of a drop of liquid can be easily aspirated and dispensed thereby.
Further, with the pipette of the present invention, the rate of upward and downward movement of the plunger unit is within the complete manual control of the pipette user. By the proper manual control of the rate of piston movements, problems associated with xe2x80x9cfountainingxe2x80x9d and the xe2x80x9caerosolsxe2x80x9d caused by too rapid movement of the plunger unit in conventional manual air displacement can be eliminated.
Still further, in the manual air displacement pipette of the present invention, the only force opposing user initiated axial movement of the plunger unit may be the piston seal which creates the necessary fluid tight seal around the piston of the plunger unit. Such seal friction is sufficient to hold the plunger unit in any axial position where it is located by the pipette user. Thus, for example, in the pipette of the present invention, once the pipette user manually moves the plunger to the home position, no further forces need be generated by the user to maintain the plunger unit at the home position.
Further, the seal friction force is so low that the upward thumb or finger force which must be generated by the pipette user to move the plunger from the home position to the upper stop position during aspiration of a selected volume of liquid or the downward force which must be generated to move the plunger unit to the lower stop position to dispense the selected volume of liquid from the pipette tip secured thereto, is absolutely minimal.
Also, during any such upward or downward plunger unit movement, the user may halt the movement of the plunger and it will remain at that location for adjustment by the user as during precision pipetting of minute liquid sample or the layering of gels or the loading of electrophoresis plates or any during any one of the several different modes of operation of the pipette, e.g. titration, measurement, multiple dispense and the like.
Like prior manual air displacement pipettes, the present invention comprises a hand holdable pipette body housing and supporting a plunger unit for axial movement from a home position to an upper stop position and between the upper stop position and a lower stop position. The home position is between the upper and lower stop positions and is the starting position to which the plunger unit is returned for the start of each successive aspiration operation with the pipette.
Also as with prior manual air displacement pipettes, once a selected volume of liquid has been aspirated into a pipette tip secured to a lower end of the pipette by upward movement of the plunger unit from the home position to the upper stop position, the pipette user presses downward on a plunger control knob secured to an upper exposed end of the plunger unit to move the plunger unit downward from the upper stop position to the lower stop position wherein the selected volume of liquid contained in the pipette tip is expelled from the tip.
With the pipette of the present invention however, such aspiration and dispensing operations are free of the continuous upward forces generated by a conventional return spring and the relatively strong upward forces generated by a conventional blow out spring. That is because the improved manual air displacement pipette of the present invention does not include either a return spring or a conventional blow out spring. Rather, in the pipette of the present invention, the only force opposing axial movement of the plunger unit may be the sliding friction force generated by a piston seal necessary to the operation of an air displacement pipette. The seal friction force may be sufficient by itself to maintain the plunger unit at any axial position selected by the pipette user. Alternatively, the piston seal force may be supplemented by an additional friction force which may be selectable by the pipette user and when combined with the seal friction force will be sufficient to maintain the plunger unit at any axial position selected by the user.
Thus, a basic embodiment of the present invention may simply include a pipette body, a plunger unit, a piston seal and means for identifying to the pipette user the location of the home position for the plunger unit between an upper and lower stop.
In other embodiments of the present invention, a weak blow out spring may be added to locate the home position of the plunger unit while in still other embodiments, mechanical or magnetic detents may be included for that purpose.
Further, is some embodiments of the present invention, magnetic or mechanical detents may be included at the upper stop position to aid in the location of the plunger unit at the upper stop.
Still further, the pipette of the present invention may be of a fixed volume pipette or an adjustable volume pipette. The adjustable volume version of the pipette may include means for adjusting the axial position of an upper stop defining the selected volume for the pipette.
Other features of the pipette of the present invention will be appreciated from a reading of the following detailed description when taken with the drawings as described below.